Haemophilus influenzae type b (Hib) is a significant pathogen for young children. Several types of vaccines (conjugate and CHO) have been produced to induce protective antibodies (Abs), Abs to the CHO capsule of Hib (Hib- PS). However, the magnitude of these human anti-Hib-PS Ab responses vary greatly (among individuals and vaccines) and anti-Hib-PS Ab may differ in affinity and protective potency. These quantitative and qualitative variabilities complicate assessment of vaccine efficacy in different populations of young children. We have demonstrated heterogeneity in the V region repertoire of anti-Hib-PS Abs by identifying their VL genes (e.g., A2, O18 VK genes). A2 gene is most commonly used. Having established the heterogeneity in the structure of individual Ab clones, we now propose to investigate how variations in function and regulation of individual Ab clones can affect host protection. To study variation in function of anti-Hib-PS clonal Abs, we will: 1) determine the variability in Ab affinity among clonal Abs, 2) compare affinity of IgG1 Ab clones induced with a conjugate vaccine in young children and adults, 3) determine variability in protective potency among clonal Abs in newborn rats, and 4) associate protective potency with Ab affinity and with VL primary structure in newborn rats. To study the regulation of the expression of individual Ab clones, we will: 1) complete the development of serological assays specific for all the different VL types of anti-Hib-PS Abs by making a monoclonal Ab specific for the anti- Hib-PS Ab using the O18 gene. Using these assays, we will then 2) determine if the three available conjugate vaccines induce different Ab repertoires in adults, 3) determine if young children express Ab V region repertoires different from adults, 4) determine the prevalence of A2 gene deletion, 5) determine if any genetic factor including the kappa chain haplotype affects Ab V region repertoire by studying a genetically well characterized population, and 6) determine if a select population expresses Ab repertoire different from others by studying Navajo Indians who are poorly responsive to Hib-PS. Our study will provide detailed knowledge of expression and function of individual clones of Abs to a CHO antigen (Ag) and an analogous protein Ag directly in human. That knowledge will be highly useful to our long term goal of studying Ag-driven human B cell maturation. Our studies should be also relevant to developing conjugate vaccines to other medically important bacteria such as S. pneumonia.